Polyester and its use in powder coating

ABSTRACT

An improved carboxyl-group-containing polyester resin composition as described of an aromatic carboxylic acid and an aliphatic diol having the following combination of properties: (a) an acid number between 10 and 26 mg KOH/g, (b) a hydroxyl number that is smaller than 15 mg KOH/g, (c) a theoretical number-average molecular weight between 4,500 and 12,500 and (d) a glass transistion temperature between 40° and 85° C.

This is a division of application Ser. No. 478,546, filed Mar. 24, 1983,now U.S. Pat. No. 4,471,108, granted Sept. 11, 1984.

This application relates to a novel carboxyl group containing polyesterwhich polyester is substantially based on an aromatic dicarboxylic acidmixture and on a substantially aliphatic diol mixture and which is usedwith a tri-epoxy compound as binding agent in powder coating. Inparticular, triglycidyl isocyanurate is advantageously used togetherwith the polyester resin in powder coatings.

Polyesters containing carboxyl groups for powder coatings are alreadyknown in the prior art, and in practice these polyesters are processedto form binding agents for powder coating in a combination with at least7% triglycidyl isocyanurate (TGIC) and consequently at most 93%polyester. Compare Dutch Patent Specification No. 159,129, as well asDutch Published Application No. 7604421.

In use, these known binding agents for powder coating present one orseveral of the following problems:

1. The relatively high content of TGIC requires a polyester having arelatively high glass transition temperature, as a result of whichlimitations in the freedom of formulation of the polyester occur.

2. The flow of the polyester-epoxy combination is not optimal("orange-peel effect").

3. The physical and sometimes also the chemical powder stability is notall that is to be desired, particularly with polyesters having a glasstransition temperature below 55° C.

4. The cured coating layer is sometimes not completely water-clear.

It is an object of this invention to provide a novel improved carboxylgroup containing polyester resin composition which has advantageouscharacteristics in overcoming the aforesaid problems; and further toprovide novel coating compositions composed of such resins incombination with tri-epoxy compounds.

The present invention now provides polyesters and binding agents forpowder coatings which are improved with respect to one or several of theaforementioned problems. The polyesters according to the inventionexhibit a combination of characteristics, namely an acid number of 10-26mg KOH/g, a hydroxyl number of at most 15, advantageously a maximum of 5mg KOH/g, a theoretical number-average molecular weight (calculatedaccording to Patton) of 4,500-12,000 and a glass transition temperaturebetween 40° and 85° C.

Especially conspicuous in such polyesters is especially the relativelyhigh molecular weight; and, it is quite surprising that in spite of thishigh molecular weight an excellent flow of the powder coating isobtained. Moreover, because of the relatively high molecular weight, thepolyester resin contains relatively little low molecular material, as aresult of which a better powder stability arises. The low lower limit ofthe glass transition temperature range is also conspicuous. The greaterclearness in combination with the excellent flow exhibited by thecoatings using the binding agents employing the polyester resin madeaccording to this invention makes these polester/epoxy systemsparticularly suitable as a top-coat for automobiles.

Preferably the acid number of this polyester (resin) is between 12 and24 mg KOH/g. The theoretical number-average molecular weight of thepolyester is preferably 5,500-10,000, while the glass transitiontemperature is preferably between 50° and 70° C. The average carboxylfunctionality of the polyester is situated in the range of 2 to 3,preferably between 2.2 and 2.8, more particularly between 2.2 and 2.5.

The polyesters containing carboxyl groups, which are provided accordingto the invention, may be prepared, by methods which are already knownper se, from substantially aromatic polycarboxylic acids, such asphthalic acid, isophthalic acid, terephthalic acid,benzene-1,2,4-tricarboxylic acid, pyromellitic acid, trimesic acid,3,6-dichlorophthalic acid, tetrachlorophthalic acid, as well as, insofaras they are obtainable, from the corresponding anhydrides, acidchlorides or lower alkyl esters thereof.

Preferably the carboxylic acid component consists of at least 50%,preferably at least 70 mol. % of aromatic dicarboxylic acids,particularly isophthalic acid and/or terephthalic acid.

Further, lower aliphatic diols may especially be used for the polyesterformation, such as ethylene glycol, propane-1,2-diol, propane-1,3-diol,butane-1,2-diol, butane-1,4-diol, butane-1,3-diol,2,2-dimethylpropanediol-1,3 (i.e., neopentyl glycol), hexane-2,5-diol,hexane-1,6-diol, 2,2-[bis-(4-hydroxycyclohexyl)]-propane,1,4-dimethylolcyclohexane, diethylene glycol, dipropylene glycol and2,2-bis-[4-(2-hydroxylethoxy)]-phenylpropane and smaller amounts ofpolyols, such as glycerol, hexanetriol, pentaerythritol, sorbitol,trimethylolethane, trimethylolpropane andtris-(2-hydroxyethyl)-isocyanurate. Instead of diols and polyols, epoxycompounds may also be used.

Preferably the alcohol component contains at least 50 mol. % neopentylglycol and/or propylene glycol.

In addition, cycloaliphatic and/or acyclic polycarboxylic acids may beused as the polycarboxylic acids, such as, for example,tetrahydrophthalic acid, hexahydroendomethylenetetrahydrophthalic acid,azelaic acid, sebacic acid, decanedicarboxylic acid, dimeric fatty acid,adipic acid, succinic acid and maleic acid. Such acids are used inamounts up to at most 30 mol. %, preferably to a maximum of 20 mol. % ofthe total amount of carboxylic acids. Similarly, hydroxycarboxylic acidsand/or alternatively lactones may be used, e.g., 12-hydroxystearic acid,epsiloncaprolactone, hydroxypivalic acid ester of neopentyl glycol(esterdiol 204). Monocarboxylic acids, such as benzoic acid,tert.-butylbenzoic acid, hexahydrobenzoic acid and saturated aliphaticmonocarboxylic acids may also be added in minor amounts during thepreparation.

The polyesters are prepared by methods already known per se, e.g., byesterification or interesterification, possibly in the presence of usualcatalysts such as e.g., dibutyl-tin oxide or tetrabutyl titanate,whereby, through a suitable choice of the reaction conditions of theCOOH/OH ratio, end products are obtained for which the acid number liesbetween 10 and 25, preferably between 12 and 23.

After synthesis, the polyesters are preferably practically free fromhydroxyl groups, i.e., the hydroxyl number is less than 15, preferablyless than 5 mg KOH/g. For optimal adjustment of the glass transitiontemperature, and the viscosity, it is also advantageous to use up to 15mol. % of a compound having 4 or more methylene groups optionallysubstituted with lower alkyl groups (methyl or ethyl). For example,adipic acid, hexanediol-1,6 or dipropylene glycol may be used as such.

As cross-linking agent preferably triglycidyl isocyanurate or a relatedheterocyclic tri-epoxy compound is used. As such, methyl-substitutedtriglycidyl isocyanurate or 1,2,4-triglycidyl-triazolidin-3,4-dione maybe used. Preferably used is TGIC having an epoxy content of at least 9.3equivalent per kg.

The amount of tri-epoxy compound that is used in combination with thepolyester containing carboxyl groups according to the invention, to formthe novel coating compositions, is dependent on the acid number of thepolyester and lies between 0.8 and 1.2, preferably between 0.9 and 1.1equivalent epoxy per equivalent carboxyl. In the case of triglycidylisocyanurate this means that between 1.4 and 5.3, preferably between 1.6and 4.9 wt. % TGIC and consequently 94.7 to 98.6, preferably 95.1 to98.4 wt. % polyester are used. It is quite surprising that, with the useof these small amounts of TGIC, nevertheless well-cured coating layersare obtained.

To make the coating compositions, the polyester and the tri-epoxycompound are homogeneously mixed, preferably in the melt, and preferablyby means of an extruder mechanism. As a rule, the usual additives areintroduced before the mixing. This especially applies to additives suchas flowing aids, curing catalysts and optionally pigments and/orfillers. The mixture thus obtained is subsequently ground and sieved,and is then applied with the aid of an electrostatic spraying apparatusand thereafter cured in a stoving oven at temperatures between 160° and200° C. for 10 to 30 minutes. The coating layer thus obtained exhibitsan excellent combination of gloss, flow, clearness and mechanicalproperties, while the powder coating has very good powder stability bothin chemical and physical respect.

The invention will be further illustrated by the following non-limitingexamples:

EXAMPLE 1A A. Preparation of the polyester resin

A three liter reaction vessel, equipped with a thermometer, a stirrerand a distillation apparatus, was filled with the following materials:17 g trimethylolpropane, 1,475 g terephthalic acid, 1,055 g neopentylglycol, 58 g 1,4-cyclohexanedimethylol, 71 g hexanediol-1,6 and 29 gadipic acid.

Thereafter, with stirring, while a light stream of nitrogen was led overthe reaction mixture, the temperature was increased to 200° C., whereatwater formed. The temperature was gradually increased further to amaximum of 250° C. and the water was distilled off. After 453 ml waterhad been collected the acid number of the ester was 13.7 mg KOH/g.

Subsequently, 355 g isophthalic acid was added and further esterified toan acid number of 19.8 mg KOH/g. The last part of this process wascarried out under reduced pressure.

The theoretical number-average molecular weight (Mn) of the acidicpolyester obtained was 6,500, the glass transition temperature was 58°C., the hydroxyl number was less than 5 mg KOH/g and the viscosity was700 dPa.s at 165° C. (measured with an Emila rotation viscosimeter).

EXAMPLES 2A-10A

Following the procedure described in Example 1A, polyester resinsaccording to the invention were prepared of which the chargecomposition, acid number, OH number, viscosity at 165° C., glasstransition temperature and theoretical number-average molecular weightare given in following Table 1.

EXAMPLE 1B B. Preparation of the powder coating

576 g of the granulated polyester resin of Example 1A was dry-mixed with24 g triglycidyl isocyanurate, 300 g titanium dioxide (from Kronos™, CL310), 9 g Resiflow™ PV-5 (a polyacrylate 66% active, remainder probablysilica--from Worlee™) and 4.5 g benzoin, and subsequently introducedinto an extruder (Buss™ made, type PR 46). The extrudate was cooled,ground and sieved, the sieve fraction, smaller than 90 micrometer, beingcollected and used as the powder coating. This powder coating waselectrostatically sprayed onto steel panels which had been degreasedbeforehand with trichloroethane. For this, a Gema™ made, type HP 720,electrostatic spraying apparatus was employed. Thereafter the panelswere evaluated on the following properties:

Glass (Gardner 60°): 92

Mechanical properties:

(Reverse Impact): greater than 11 kg/cm² (i.e., greater than 160 psi)

Flow of the coating layer visual: 10

Powder stability (28 days at 40° C.): good.

EXAMPLES 2B-10B

In the same manner as in Example 1B, powder coatings were preparedstarting from the polyester resins according to Examples 2A-10A. Thecompositions and the test results of these powder coatings are given infollowing Table 2.

EXAMPLE 11B

Starting from the polyester resin 7A, a clear (non-pigmented) powdercoating was prepared following the same procedure as described inExample 1B, with the exception of the fact that 4.8 g silicon surfactantSR 232 (from British Petroleum) was used as the flowing aid instead ofResiflow™ PV-5. The panels sprayed therewith exhibited propertiessimilar to those of Example 7B, while the coating layer had excellentclearness.

                                      TABLE 1                                     __________________________________________________________________________    Composition and properties of the exemplified polyesters                                   2A   3A   4A   5A   6A   7A   8A   9A   10A                      __________________________________________________________________________    Composition (g.)                                                              terephthalic acid                                                                          1669 1727 1499 1709 1530 1518 1550 1445 1605                     neopentyl glycol                                                                           1030 1004 1040 1065 1029 1002 1017 1013 1079                     adipic acid                                     29   83                       hexanediol-1,6                                                                             71                  110  121       93   84                       pentanediol-1,5        63   84                                                diethylene glycol                          110                                isophthalic acid                                                                           155  154  372  171  325  341  314  348  113                      1,4-cyclohexanedimethylol                                                                  58        58        54   54   50   57                            ethylene glycol   99                                                          trimethylolpropane                                                                         17   17   30   30   12   25   19   17   16                       12-hydroxystearic acid                                                                     60   59                                 21                       Properties                                                                    acid no. (mg KOH/g)                                                                        19.8 18.0 20.2 20.5 16.4 20.8 19.8 19.6 16.5                     Mn           6500 6500 6500 6500 7000 7000 7500 6500 9000                     Hydroxyl no. (mg KOH/g)                                                                    <5   <5   <5   <5   <5   <5   <5   <5   <5                       Viscosity at 165° C. (dPa.s)                                                        430  720  1100 1300 650  1100 970  610  510                      Glass transition temp (°C.)                                                         52   56.5 57.5 58   59.5 58.5 56   54   48.5                     Average carboxyl func-                                                                     2.3  2.3  2.5  2.5  2.2  2.5  2.4  2.3  2.4                      tionality                                                                     __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Composition and properties of the exemplified powder coatings                               2B  3B  4B  5B  6B  7B  8B  9B  10B                             __________________________________________________________________________    Composition (g.)                                                              Polyester resin                                                                             576 576 576 576 582 576 576 576 583                             TGIC*         24  24  24  24  18  24  24  24  17                              Pigment (TiO.sub.2)                                                                         300 300 300 300 300 300 300 300 300                             Resiflow PV-5 9   9   9   9   9   9   9   9   9                               Benzoin       4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5                             Properties                                                                    Gloss (Gardner 60°)                                                                  92  90  91  93  90  95  88  91  92                              Mech.prop.** (kg/cm.sup.2) (psi)                                                            11  11  8   9   11  11  11  11  11                              Flow***       10  9   8   8   9   10  10  10  10                              Powder stability                                                                            good                                                                              good                                                                              good                                                                              good                                                                              good                                                                              good                                                                              good                                                                              good                                                                              good                            (28 days, 40° C.)                                                      __________________________________________________________________________     *triglycidyl isocyanurate                                                     **determined according to ASTM D 279469                                       ***judged visually on a scale of 1-10, in which                               1 = very bad                                                                  10 = excellent                                                           

We claim:
 1. A powder coating wherein the binding agent is composed of ahomogeneous mixture of a tri-epoxy compound and a polyester containingcarboxyl groups, wherein said mixture contains between 1.4 and 5.3 wt. %of a tri-epoxy compound, and from 94.7 to 98.6 wt. % of a polyester ofan aromatic dicarboxylic acid and an aliphatic diol having the followingcombination of properties: (a) an acid number between 10 and 26 mgKOH/g, (b) a hydroxyl number that is smaller than 15 mg KOH/g, (c) atheoretical number-average molecular weight between 4,500 and 12,500 and(d) a glass transition temperature between 40° and 85° C.
 2. A polyesteraccording to claim 1, wherein said acid number is between 12 and 24 mgKOH/g.
 3. A polyester according to claim 1 wherein said hydroxyl numberis 5 mg KOH/g at a maximum.
 4. A polyester according to claim 1, whereinsaid theoretical number-average molecular weight is between 5,000 and10,000.
 5. A polyester according to claim 1, wherein said glasstransition temperature of the polyester is between 50° and 70° C.
 6. Apolyester according to claim 1, wherein said average carboxylfunctionality of the polyester is between 2 and
 3. 7. A polyesteraccording to claim 1, wherein said average carboxyl functionality of thepolyester is between 2.2 and 2.8.
 8. A polyester according to claim 1,wherein said dicarboxylic acid mixture consists of at least 50 wt. % ofisophthalic acid and/or terephthalic acid.
 9. A polyester according toclaim 1, wherein said dicarboxylic acid mixture consists of at least 70mol. % of isophthalic acid and/or terephthalic acid.
 10. A polyesteraccording to claim 1, wherein said aliphatic diol mixture consists of atleast 50 mol. % of neopentyl glycol and propylene glycol.
 11. Apolyester according to claim 1, wherein said polyester contains acompound which has at least 4 methylene groups in an amount of up to 15mol. %.
 12. A powder coating according to claim 10, wherein the weightpercentage of said tri-epoxy compound is between 1.6 and 4.9.
 13. Apowder coating according to claim 10 wherein said tri-epoxy compound istriglycidyl isocyanurate.
 14. A polyester according to claim 1 whereinsaid acid number is between 12 and 24 mg KOH/g, said hydroxyl number is5 mg KOH/g at a maximum, said theoretical number-average molecularweight is between 5,000 and 10,000, said glass transition temperature ofthe polyester is between 50° and 70° C. and said average carboxylfunctionality of the polyester is between 2 and
 3. 15. A polyesteraccording to claim 14 wherein said dicarboxylic acid mixture consists ofat least 50 wt. % of isophthalic acid and/or terephthalic acid.
 16. Apolyester according to claim 14 wherein said dicarboxylic acid mixtureconsists of at least 70 mol. % of isophthalic acid and/or terephthalicacid.
 17. A polyester according to claim 14 wherein said aliphatic diolmixture consists of at least 50 mol. % of neopentyl glycol and propyleneglycol.